1. Field of the Invention
The present invention relates to a surface mount quartz crystal oscillator, and more particularly, to a surface mount crystal oscillator which has terminals for testing disposed on an outer surface thereof.
2. Description of the Related Art
A surface mount crystal oscillator which accommodates a quartz crystal unit and an oscillator circuit using the crystal unit in a surface mount package has been widely used, particularly for portable electronic devices as a frequency or a time reference source because of its small size and light weight. In recent years, an increasing reduction in size has been rapidly advanced in a variety of portable devices represented by portable telephones, causing an associated reduction in size in surface mount crystal oscillators. For example, crystal oscillators having the external dimensions of 5 mm×3.2 mm, and 3.2 mm×2.5 mm are currently available on the market, and an announcement has been made on the development of a crystal oscillator having the external dimensions of 2.5 mm×2 mm. However, with an increasing reduction in size, several disadvantages arise in surface mount crystal oscillators. One of such disadvantages is a problem of terminals provided on an outer surface of the crystal oscillator for purposes of testing or adjustments.
A crystal oscillator is provided with a power supply terminal, a ground terminal, and an output terminal for use in a normal operation, and is also provided with terminals for use in testing and adjustments which may be made during manufacturing or at the time of shipment. The terminals for testing or adjustments are typically disposed on an outer side surface of a surface mount crystal oscillator. Japanese Utility Model Laid-open application No. 5-65110 (JP, 5-65110, U), for example, discloses a crystal oscillator which has terminals directly connected to a quartz crystal blank as testing terminals. However, an increasing reduction in size of the crystal oscillator has imposed more limitations in disposing the terminals for testing or adjustments.
FIG. 1A is a partially cut-away front view illustrating a conventional surface mount crystal oscillator. FIGS. 1B and 1C are plan views of the crystal oscillator illustrated in FIG. 1A when a cover has been removed from the crystal oscillator, and when the cover is put on the crystal oscillator, respectively.
The surface mount crystal oscillator employs a package body 1 formed with a recess which receives IC (integrated circuit) chip 2 and quartz crystal blank 3, and cover 4 which is placed over the recess for closing the same to hermetically encapsulate IC chip 2 and crystal blank 3 within package body 1. Package body 1, which is made of laminated ceramics, is comprised of bottom wall 5 and first frame wall 6 laminated on bottom wall 5. Bottom wall 5 is comprised of flat plate 7 and second frame wall 8 laminated on flat plate 7. Flat plate 7, first frame wall 6, and second frame wall 8 correspond to respective ceramic green sheets, where first frame wall 6 is formed with a central opening extending therethrough substantially in the shape of rectangle, and second frame wall 8 is formed with a central opening extending therethrough substantially in the shape of rectangle smaller than the central opening of first frame wall 6. By thus designing bottom wall 5 and first frame wall 6, a step is formed in the recess of package body 1.
At positions on the top of flat plate 7 corresponding to the bottom of package body 1, IC terminals, not shown, are formed for use in connection with IC chip 2. Also, at positions on the top of second frame wall 8 corresponding to the step in the recess of package body 1, crystal terminals 9 are formed for use in connection with crystal blank 3. Package body 1 is further formed with a conductor pattern, not shown, for connecting the IC terminals to crystal terminals 9.
IC chip 2, which comprises an integrated oscillator circuit including a temperature compensation function, is secured to the IC terminals on the bottom of the recess in package body 1, for example, by flip chip bonding. Crystal blank 3 is, substantially in the shape of rectangle and, for example, an AT-cut quartz crystal blank. Crystal blank 3 is provided with an excitation electrode, not shown, on each of main surfaces thereof, and extending electrodes (not shown) are extended from the excitation electrodes toward a pair of opposing sides. Then, crystal blank 3 is secured by conductive adhesive 21 to the step in the recess of package body 1, i.e., crystal terminals 9 formed on second frame wall 8 in outer peripheral regions of both sides of crystal blank 3 to which the extending electrodes are extended.
Cover 4, which is made of a metal, is bonded to a metal ring or a metal thick film, not shown, provided on the top of package body 1, i.e., the top of first frame wall 6 by seam welding, thereby enclosing the recess of package body 1 to hermetically encapsulate IC chip 2 and crystal blank 3 within the recess.
At four corners on the bottom of package body 1, i.e., at the four corners of the bottom of flat plate 4, mounting electrodes 20 are formed for use in surface-mounting the crystal oscillator on a wiring board. Mounting electrodes 20 thus formed also extend off outer side surfaces of package body 1. Mounting electrodes 20, which will serve as a power supply terminal, a ground terminal, an output terminal, and the like of the crystal oscillator, are connected to IC chip 2 through conductive paths and IC terminals, not shown.
In such a surface mount crystal oscillator, four write terminals 10a and a pair of testing crystal terminals 10b, for example, are provided on the outer side surfaces of second frame wall 8, for example, of package body 1 for use in adjustments and testing. Write terminals 10a are used for writing temperature compensation data for the temperature compensation mechanism within IC chip 2. The temperature compensation data is determined based on a change in oscillating frequency due to the temperature by measuring the oscillating frequency from the crystal oscillator which is operated while the ambient temperature is varied. By writing the temperature compensation data, the temperature compensation mechanism normally operates, thus permitting the crystal oscillator to function as a temperature compensated crystal oscillator (TCXO). Testing crystal terminals 10b are used for testing, for example, CI (crystal impedance) and resonance characteristic of the crystal unit (crystal blank 3) alone, for example, after it is encapsulated by cover 4.
These write terminals 10a and testing crystal terminal 10b are formed by extending the IC terminals and crystal terminals 9 from the laminated surface to the outer side surfaces of second frame wall 8, in other words, by the through-holes formed through the ceramic package. For writing data or making a measurement using write terminals 10a and testing crystal terminals 10b, a needle-shaped device or jig called “probe” is brought into contact with these terminals. For permitting the probe to appropriately come into contact with the terminals, vertically extending cavities are formed in first frame wall 6, second frame wall 8, and flat plate 7 at positions at which these terminals are formed, and the terminals are formed on the surfaces of the cavities in second frame wall 8.
However, with the increasing reduction in size of the crystal oscillator in the foregoing structure, a resulting reduced area of the outer side surface of the package makes it more and more difficult to form as many write terminals 10a and testing crystal terminals 10b as required on the side surface of package body 1. Also, since write terminals 10b, testing crystal terminals 10b, and mounting terminals 20 are in closer proximity to one another, solder can introduce around write terminals 10b and testing crystal terminals 10b, when the crystal oscillator is mounted on a wiring board by reflow soldering or the like, thus possibly causing malfunctions. Further, since through-holes are formed in the cavities cut into the side surfaces of package body 1 in order to form write terminals 10a and testing crystal terminals 10b, the side wall of package body 1 has locally smaller widths where the cavities are formed, resulting in a problem of partially lower mechanical strengths. As a result, during seal welding of cover 4, the through-holes can be damaged.